The present invention generally relates to sensing devices and sensing techniques thereof. The present invention also relates to magnetic sensors. The present invention additionally relates to printed circuit board (PCB) and small outline integrated circuit (SOIC) devices and components utilized in magnetic sensor packages. The present invention also relates to techniques for calibrating magnetic sensors.
A variety of magnetic sensors are known in the art. Such magnetic sensors are utilized in automotive applications, including, but not limited to, camshaft, crankshaft and rotary position sensors. Such sensors often are configured to include Hall effect or magnetoresistive (MR) transducers, which are typically combined within magnetic or ferromagnetic targets having a pattern of features keyed to mechanical details being monitored by the magnetic sensor. These features represent the magnetic signature of a magnetized target or the presence or absence of ferromagnetic metal in various sizes on a metal target placed near an associated transducer.
Semiconductor Hall elements and magnetoresistors, in particular, are widely used in magnetic sensing applications. Such xe2x80x9cHallxe2x80x9d elements are based on the Hall effect, well known in the art, which arises from the Lorentz force acting on the charge carriers in a conductive material. This phenomenon arises as follows: given a rectangular thin plate of conductive material with an electric potential applied along the Y axis and a magnetic field applied perpendicular to the plate (along the Z axis), the Lorentz force is represented by the formulation F=q(vxc3x97B), such that the variable q represents the charge of the carrier (usually electrons), the variable v represents the velocity of the charge carrier, and the variable B represents the magnetic field. The force distorts the current flow and crowds it toward one side of the conductive plate. This phenomenon distorts the equipotential lines and generates a Hall voltage.
The Hall voltage is proportional to xcexcB, wherein the variable xcexc represents the mobility of the material, and the variable B represents the magnitude of the magnetic field. Pickoff points for the Hall signal (Vo1-Vo2) are usually located at the midpoint of the plate along the Y axis. In long plates, a Hall electric field balances the Lorentz force, and the current flow becomes parallel to the Y axis, driving the Hall voltage to zero. Most practical Hall elements are roughly square and the current flow is at an angle with respect to the excitation voltage. This is what gives rise to the geometric magnetoresistance effect, which is further described below.
The fact that the current must travel further in a short Hall plate when a magnetic field is applied causes an increase in resistance. This phenomenon is referred to as the geometric magnetoresistance effect. Magnetoresistor (MR) elements typically have their length less than or equal to their width. A long resistor would thus not exhibit a magnetoresistance effect. Practical sensors generally utilize a large number of elements in series to increase the insertion resistance. The geometric magnetoresistance effect is proportional to xcexc2B2 for small fields (i.e., where xcexc represents the mobility) and thus requires a significant magnetic bias to obtain a useful signal. The change in resistance is identical for a positive or negative field of the same magnitude.
Magnetic sensors are generally utilized to sense the position or location of a particular target. When a metal target is circular in shape, for example, the sensing mechanism may be referred to as a xe2x80x9cgear tooth sensorxe2x80x9d because of the resemblance of the target to a toothed mechanical gear. These gear tooth sensors are often used in the automotive arts in situations in which the target is linked to a crankshaft for use in engine control.
As a result of government regulations and the desire by automobile manufactures for the ability to provide misfire detection in automobile engines, the required accuracy and repeatability of automotive gear tooth sensors have been steadily increasing in recent years. In combination with these increasing requirements, operating conditions of gear tooth sensors now include increased air gap dimensions and axial run-out conditions. Additionally, larger effective magnetic signals are required to improve the signal-to-noise ratio of the device.
The magnitude of the effective magnetic signals in a gear tooth sensor can be increased by increasing the size and strength of the permanent magnet or, alternatively, by decreasing the distance between the permanent magnet and the target. If the size and strength of the magnet are increased, the overall costs of the gear tooth sensor will also be increased. A less expensive method for producing larger magnetic signals involves the design of a package for the gear tooth sensor that can minimize the distance between the permanent magnet and the ferromagnetic target. Such a reduction in the distance between the magnet and the target can also permit smaller permanent magnets to be utilized at a reduced cost.
With regard to the characteristic that permits the magnet to be placed closer to the target, it should be understood that in the past gear tooth sensors have been constructed with small outline integrated circuit (SOIC) component packages. These structures are typically disposed on an electrical substrate, such as a flex circuit or printed circuit board (PCB). The permanent magnet is disposed beneath the SOIC component and is usually separated from it by some type of plastic thickness.
In a typical prior art gear tooth sensor package of this type, the resulting distance between the pole face of the permanent magnet and the target comprises the plastic thickness on which the electrical substrate is mounted. Such a resulting distance also can determine the thickness of the electrical substrate (e.g., printed circuit board, flex circuit) and the SOIC thickness, which includes the stand-off height of the leads, the bottom plastic thickness of the SOIC, the lead frame thickness, the die thickness, the wire bond maximum height, the clearance above the wire bond maximum height, and the top plastic thickness of the SOIC.
Based on the foregoing, it can be appreciated that sensor designers continually seek refinement of the target system to improve engine control. Many sensing applications require a small diameter probe sensor with a short mounting profile from an associated mounting flange to a spinning ferrous target. Some application specifications require a calibrated magnetic circuit to ensure proper performance at all times and under all possible conditions. Devices based on such applications may also require sealing for an automotive under-the-hood environment and to provide an O-ring type seal to the engine. Having an O-ring configuration incorporated into the sensor reduces the working diameter of the internal sensor construction.
The present inventors have determined that a process issue associated with such prior art designs that must be overcome involves the use of integrated-circuit (IC) packages that may be mounted 90 degrees to the plane of the printed circuit board. Such a configuration requires a much more difficult through-hole soldering process that results in a greater amount of reworking, which in turn affects cost and efficiency. The present inventors have thus concluded that a solution to these issues involves the creation of a new design that utilizes an SOIC package that eliminates the need for a secondary through-hole soldering process to attach the IC. According to the present invention, an SOIC solder attachment results in a more robust process and eliminates the solder rework created by the through-hole solder process.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide an improved magnetic sensor.
It is another aspect of the present invention to provide a calibrated, low-profile magnetic sensor.
It is still another aspect of the present invention to provide a magnetic sensor assembly package that eliminates the need for secondary through-hole soldering to attach a compact integrated circuit package.
It is yet another aspect of the present invention to provide a method for forming a calibrated low-profile magnetic sensor.
The above and other aspects can be achieved as is now described. A calibrated, low-profile magnetic sensor and method for forming such a sensor are described herein. Generally, a magnetic sensing device is formed within a housing, wherein the magnetic sensing device comprises at least one magnet. The magnetic sensor includes a compact integrated circuit package such as, for example, a small outline integrated circuit package (SOIC). A magnetic sensing element is generally mounted in the compact integrated circuit package. The magnetic sensing device can be configured to additionally incorporate a printed circuit board (PCB) having a hole formed therein, such that the compact integrated circuit package can be surface mounted off-center on the PCB, so that the hole is generally located within the printed circuit board to match a shape of the magnet, thereby allowing the magnet to pass through the hole so as to be placed adjacent to the compact circuit package, completing the magnetic circuit. An orifice can be built into the housing of the magnetic sensor with the axis of the orifice located coincidentally with the magnet. A tool can be passed through the orifice within the housing to contact the magnet. The tool is generally used to push the magnet into proper position for calibration optimization of the magnetic circuit thereof.
Thus, a magnetic sensing element can be mounted in an SOIC package, which in turn is surface mountable to a PCB. The SOIC is mounted off center on the PCB so that a hole can be placed within the PCB that fits the shape of the magnet. Such a magnet can be configured as a small outline magnet, which passes through the hole in the PCB adjacent to the SOIC to complete a magnetic circuit. The magnet is mounted such that the position of the magnet can be adjusted to provide the optimum magnetic circuit. An orifice can be placed within the magnetic sensor to gain access to the magnet to push it into proper calibration position. This same orifice also provides a port for the purpose of injection dispensing of epoxy used to encapsulate and seal the device. By the nature of using the same orifice for encapsulation and calibration of the magnet, the need for a secondary potting process to seal the calibration port is eliminated.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.